1. Field of the Invention
The present invention relates to ferroelectric thin-film devices and methods for making the same, and particularly relates to a ferroelectric thin-film device, which is applicable to not only capacitors for DRAM (dynamic random access memory) and ferroelectric RAM but also pyroelectric elements, microactuators, thin film capacitors and compact piezoelectric elements, and relates to a method for making the same.
2. Description of the Related Art
Recently, the formation of epitaxial thin films on single crystal substrates has been intensively studied regarding lead-based perovskite compounds, such as PbTiO.sub.3, (Pb,La)TiO.sub.3 (Pb.sub.1-x La.sub.x TiO.sub.3, hereinafter abbreviated as PLT), Pb(Zr,Ti)O.sub.3 (PbZr.sub.y Ti.sub.1-y O.sub.3, hereinafter abbreviated as PZT), (Pb,La) (Zr,Ti)O.sub.3 (Pb.sub.1-x La.sub.x Zr.sub.y Ti.sub.1-y O.sub.3, hereinafter abbreviated as PLZT) and Pb(Mg,Nb)O.sub.3 (PbMg.sub.z Nb.sub.1-z O.sub.3, hereinafter abbreviated as PMN). This is because when a lead-based perovskite compound having large residual polarization, such as PZT or PLZT, is epitaxially grown, spontaneous polarization can be unified in one direction so that larger polarizability and switching characteristics are achieved, and application to high-density recording media will become promising.
Uses in which such spontaneous polarization is unified in one direction along the thickness, however, requires a metal-ferroelectric-metal (MFM) structure in which the ferroelectric thin film is sandwiched between conductive (electrode) layers on a substrate.
Generally, when a thin film of a lead-based perovskite compound is formed, lead is likely evaporated from the deposited thin film during its formation. As a result, the composition of the thin film often deviates from the intended composition. In order to cope with the problem, lead is supplied in an amount which is larger than the stoichiometric ratio in raw materials in the method for preventing such a change in composition. However, according to this method, excess Pb is oxidized to form PbO. Since the lattice constant of PbO is substantially equal to the lattice constant of the lead-based perovskite compound, PbO also is epitaxially grown under the conditions for epitaxial growth of the lead-based perovskite compound. Thus, the epitaxial growth of the lead-based perovskite compound is inhibited, resulting in deterioration of crystallinity and the morphology of the surface.